When a high load operation is continued in a motor having a permanent magnet, it is important to protect the permanent magnet from being overheated by a rise in the temperature of the motor. When the temperature of the magnet excessively rises, the magnetic force of the permanent magnet irreversibly decreases, so that the magnetic force does not return to the original force even when the magnet temperature returns to normal temperature. Thus, a torque obtained from the synchronous machine when the same current is applied thereto, is decreased by an amount equivalent to an amount by which the magnetic force is decreased. In order to avoid such a situation, the permanent magnet needs to be protected from being overheated by a rise in the temperature of the motor.
Methods for protecting the permanent magnet from being overheated are roughly classified into: a method in which designing is performed with an allowance provided in advance to beat radiation such that the permanent magnet is not overheated even in any operating state; and a method in which the magnet temperature is detected, and during heating, operation is controlled by reducing an operation load or activating a cooler.
Furthermore, methods for detecting the magnet temperature in the method in which operation is controlled, are roughly classified into: a method in which a temperature sensor or a sensor for detecting a magnetic flux which changes in response to the magnet temperature is additionally installed for detecting the magnet temperature; and a method in which the magnet temperature is estimated from a current, a voltage, or the like during operation of the motor.
In addition, examples of operation in which the magnet temperature easily rises in the method in which the magnet temperature is estimated from a current, a voltage, or the like during operation of the motor, include not only a state where the motor is rotated at a high speed with a large torque by a high current, but also a state where three-phase short circuit operation of the motor is performed. Here, the three-phase short circuit operation is operation in which three-phase terminals of the motor is virtually short-circuited.
As a conventional method for estimating the magnet temperature during the three-phase short circuit operation, a method is known in which a current detection value in a three-phase short circuit state is converted to values of a d-axis coordinate and a q-axis coordinate, an induced voltage constant is obtained from a predetermined d-axis inductance and the d-axis current resulting from the conversion, and the induced voltage constant is converted to a magnet temperature (for example, see Patent Document 1).
For reference, as a method for estimating the magnet temperature in a state other than the three-phase short circuit operation, there are the following two methods.
In the first methods the magnet temperature is estimated on the basis on information about how the ratio of a magnetic flux fundamental and harmonics change as compared to that at a reference time (for example, see Patent Document 2).
In the second method, the magnet magnetic flux or the magnet temperature is estimated from the difference between an actual voltage and a voltage calculated from a motor model held within a controller (for example, see Patent Document 3 and Patent Document 4).